Added basic filter documentation.
Added basic filter documentation.

juchi/DoctrineMongoDBBundle,0mars/DoctrineMongoDBBundle,alcaeus/DoctrineMongoDBBundle,jahller/DoctrineMongoDBBundle,tomasbro/DoctrineMongoDBBundle,Soullivaneuh/DoctrineMongoDBBundle,sarcher/DoctrineMongoDBBundle,jahller/DoctrineMongoDBBundle,0mars/DoctrineMongoDBBundle,castarco/DoctrineMongoDBBundle,alcaeus/DoctrineMongoDBBundle,juchi/DoctrineMongoDBBundle,Soullivaneuh/DoctrineMongoDBBundle,doctrine/DoctrineMongoDBBundle,malarzm/DoctrineMongoDBBundle,tomasbro/DoctrineMongoDBBundle,malarzm/DoctrineMongoDBBundle,stof/DoctrineMongoDBBundle,castarco/DoctrineMongoDBBundle